Microstructural characterization and inductively coupled plasma-reactive ion etching resistance of Y2O3–Y4Al2O9 composite under CF4/Ar/O2 mixed gas conditions

In the semiconductor manufacturing process, when conducting inductively coupled plasma-reactive ion etching in challenging environments, both wafers and the ceramic components comprising the chamber’s interior can be influenced by plasma attack. When ceramic components are exposed to long-term plasma environments, the eroded components must be replaced. Furthermore, non-volatile reactants can form and settle on semiconductor chips, acting as contaminants and reducing semiconductor production yield. Therefore, for semiconductor processing equipment parts to be utilized, it is necessary that they exhibit minimized generation of contaminant particles and not deviate significantly from the composition of conventionally used Al2O3 and Y2O3; part must also last long in various physicochemical etching environment. Herein, we investigate the plasma etching behavior of Y2O3–Y4Al2O9 (YAM) composites with a variety of mixing ratios under different gas fraction conditions. The investigation revealed that the etching rates and changes in surface roughness for these materials were significantly less than those of Y2O3 materials subjected to both chemical and physical etching. Microstructure analysis was conducted to demonstrate the minimization of crater formation. Mechanical properties of the composite were also analyzed. The results show that the composite can be commercialized as next-generation ceramic component in semiconductor processing equipment applications.


Plasma etching test
To elucidate the plasma etching characteristics of the Y 2 O 3 -YAM composite across various mixed gas ratio environments, all samples were produced with uniform dimensions of 15 mm diameter and 1 mm thickness.Additionally, for comparative analysis of plasma etching behavior, commercial c-axis sapphire and Y 2 O 3 polycrystalline ceramics were prepared.The Y 2 O 3 samples were sourced from FineTech Co, Ltd. (Korea); the measured relative density of the Y 2 O 3 ceramics was 99.4%.To accurately identify post-etching changes of Y 2 O 3 and Y 2 O 3 -YAM polycrystalline ceramics, it was necessary to reduce the initial surface roughness to a level of 5 nm or less, so the surface was polished and chemical-mechanically planarized (CMP).The polished samples then underwent partial coverage with a shadow mask composed of a nickel-cobalt alloy, manufactured through the electroforming process.This shadow mask is mechanically flexible, reusable and full-surface contactable, so that it can be applied in actual plasma etching process.Therefore, this material was utilized as the shadow mask to create an environment similar to the actual process.The selectively exposed area had a length of 6 mm and a width of 1 mm.The thickness of the employed mask was 0.05 mm.Details of the plasma etching conditions and a simple schematic illustration of the plasma etching test chamber system are presented in Table 2.For the plasma test, a 13.56 MHz power supply was connected to a planar-type antenna.The input ICP power and RF bias voltage were 1.5 kW and 600 V, respectively.Under the specified discharge conditions, the plasma was sustained in inductive mode; this is a well-established approach in industrial semiconductors 37 .Plasma tests were executed within an RF-biased inductively coupled plasma (ICP) chamber, employing a gas mixture of CF 4 , O 2 and Ar 38 .The plasma density measured by a microwave cut-off probe, was 9 × 10 10 cm -3 .Exposure of all ceramic samples to the plasma environment was carried out for a duration of 1 h.The experimental pressure was 20 mTorr.To elucidate the physicochemical etching behavior of materials, the proportions of CF 4 :Ar:O 2 gases were systemically adjusted at the following ratios: 40:10:10, 30:20:10, 20:30:10, and 10:40:10 sccm.In this study, the O 2 flow rate was maintained at 10 sccm, with variation made solely to the CF 4 to Ar gas ratios to control the physical and chemical etching.The plasma gas composition ratios used in this experiment are shown in Table 3.

Characterization
Phase analysis of the sintered Y 2 O 3 -YAM composite ceramics was conducted utilizing an X-ray diffraction analysis equipment (XRD, D/Max 2500, Rigaku) with CuK α radiation, employing a scan rate of 5°/min within   Surface roughness and 3D topography of samples were further investigated through atomic force microscopy (AFM, XE-100, Park Systems), with the surface roughness (R a ) derived from measurements over 25 × 25 µm area.Microstructural images and EDS analyses of specimens were carried out using a field emission-scanning electron microscope (FE-SEM, JSM-7800F, JEOL).The Vickers hardness was measured using a Vickers hardness tester (HM200, Mitutoyo) with a 1 kg load applied to the unetched surface.

Results and discussion
Characterization of sintered Y 2 O 3 -YAM nanocomposites Figure 1b shows the measured and relative densities of the Y 2 O 3 -YAM composite sintered at 1500°C.YAM has a lower density (4.56 g/cm 3 ) than that of Y 2 O 3 (5.01g/cm 3 ), so when the volume fraction ranged from 10 to 90%, the measured density exhibited a gradual decline from 4.97 to 4.61 g/cm 3 .With increased percentage of YAM phase, despite a slight decrease in relative density from nearly 100% theoretical density to 99.4%, overall, well-densified specimens with minimal residual pores were obtained.In comparison to YAM, which typically necessitates sintering temperatures as high as 1800°C and high pressure, or Y 2 O 3 single-composition ceramics, which require a temperature of 1600°C and applied pressure, the composite of the two compositions facilitated the production of high-density specimens at lower temperatures 35,40,41 .In addition, when compounding between Y 2 O 3 and YAG compositions, densification was difficult due to reactant formation, including YAM and YAP phases; however, in this study, by compounding Y 2 O 3 and YAM, we were able to solve this problem and achieve high density.The process also permitted examination of the plasma resistance properties among specimens of nearly equivalent density.
SEM microstructural images of Y 2 O 3 -YAM composite with various volume ratios after consolidation at 1500°C for 2 h are represented in Fig. 2. Overall, dense specimens with few pores were achieved for all compositions after hot-pressing.The average grain sizes were 0.90 ± 0.53µm, 1.02 ± 0.44µm, 0.86 ± 0.41µm, 1.62 ± 1.02µm, and 0.97 ± 0.44µm from YAM1 to YAM9, in order, indicating an overall submicron meter size.In Fig. 2a-c, YAM1, 3, and 5 had a relatively fine grain size, with a small amount of nanopores at the triple pints, which could be the point of crater formation during plasma etching.On the other hand, the YAM7 specimen in Fig. 2d showed a relatively large grain size.It can be seen that due to the rapid grain coarsening during the sintering, pores are trapped within the grain without being able to escape.These intragranular pores are more difficult to eliminate than pores at grain boundaries or triple points.Figure 4 illustrates the resultant etching depths of the materials, delineating the impacts of varying plasma gas ratios of CF 4 to Ar. Sapphire, the reference, showed a fast etching rate of more than 1000 nm/h under all conditions, and the etching depth decreased as the amount of CF 4 gas decreased because the effect of chemical etching weakened because of the low density of CF 2 radicals in the gas mixture 42 .For Y 2 O 3 ceramics, the etching depth increased slightly as the amount of Ar gas increased; it then decreased because the effect of physical etching increased when amount of Ar gas increased and the fluorinated layer generated on the surface of Y 2 O 3 was easily eliminated 10 .Overall, the Y 2 O 3 -YAM composites exhibited slower etching rates compared to the two reference materials, particularly when materials were subjected to CF 4 :Ar:O 2 ratios of 20:30:10 and 30:20:10, where simultaneous physicochemical etching was applied.In these conditions, all compositions had inductively coupled plasma-reactive ion etching resistance superior to that of Y 2 O 3 .The overall etching rates of Y 2 O 3 -YAM composites increased with increases in proportion of Ar in the plasma mixture gases.Moreover, the rise in etching rate was more pronounced for higher YAM compositions, notably in the case of the YAM9 composition, the highest YAM contents.As the CF 4 :Ar:O 2 ratios varied from 40:10:10 to 10:40:10, the etching depth of YAM9 exhibited a nearly threefold increase, increasing from 101 to 283 nm per hour, surpassing the value of Y 2 O 3 .Similarly, the YAM1 composition with 10% YAM content demonstrated an increase in etching depth from 96 to 138 nm.This phenomenon can be attributed to the lower boiling point (1275 °C) of AlF 3 , which formed on the YAM composition surface through its reaction with F − separated from CF 4 gas.This value is substantially lower than the boiling point of YF 3 (2230 °C), making it challenging to produce AlF 3 on the surface.Additionally, compared to YF 3 (161 eV), the lower binding energy (77 eV) of AlF 3 renders it more susceptible to removal by physical ion bombardment, thereby influencing the etching rate 43 .
To further analyze the plasma resistance properties of the composites, surface roughness changes before and after plasma etching, according to the variety of mixed gas ratios and compositions, were investigated.The results are presented in Fig. 5.In Fig. 5a, the Y 2 O 3 single component ceramic demonstrates a substantial change in surface roughness following plasma etching across all gas composition environments.Especially, in the case of CF 4 :Ar:O 2 gas with a ratio of 10:40:10, the surface roughness ( R a ) increased significantly from 4.2 to 62.1 nm (14.8 times).The AFM 3D images in Fig. 5b,c reveal a smooth surface pre-etching, transforming at post-etching into one with large craters, each several micrometers in size.These craters emerged as a consequence of localized and intense impacts on residual pores in the microstructure of the Y 2 O 3 sintered body by Ar + ion sputtering, resulting in deterioration of the specimen surface 5,6 .The ceramic surface's irregularities directly affect the release of sputtered neutral atoms and contaminants formation.Elevated irregularities contribute to an increased release of them, potentially giving rise to the generation of particles that are inadequately evacuated 8,44 .Compared to Y 2 O 3 , all Y 2 O 3 -YAM composite had a lower roughness than Y 2 O 3 after plasma exposure.AFM 3D images before and after etching for these compositions (Fig. 5d-g) distinctly show the discrepancy in plasma etching rates.In cases in which Y 2 O 3 and YAM compositions were combined at 50:50 volume ratio, changes in surface roughness   www.nature.com/scientificreports/with plasma etching were minimal.When the CF 4 :Ar:O 2 gas mixture ratio was 10:40:10, the surface roughness change increased by a mere 1.9 times, from 2.9 to 5.4 nm.This aligns with previous findings on surface roughness change after plasma etching of nanocomposites, suggesting its efficacy in minimizing the formation of large craters.It is well-known that changes of surface roughness are significantly dependent on grain size 26 .Therefore, the positive effect in this study can be attributed to the inhibition of grain growth through a pinning effect and densification achieved by reducing the sintering temperature 25,45 .www.nature.com/scientificreports/ The SEM images presented in Fig. 6 show microstructures of Y 2 O 3 and Y 2 O 3 -YAM composites after plasma etching test conducted with CF 4 :Ar:O 2 gas ratio of 40:10:10.In Fig. 6a, the Y 2 O 3 polycrystalline ceramic exhibited the development of substantial craters, each several micrometers in size, after plasma etching, covering the entire specimen.As previously mentioned, the emergence of these craters is associated with pronounced etching of micropores within the specimen, particularly in open pores; the resultant large craters can induce noteworthy alterations in surface roughness.Contrastingly, in Fig. 6b-f, for the Y 2 O 3 -YAM composite, dark regions signify YAM composition, while light regions denote Y 2 O 3 composition, revealing an evident discrepancy in etching rate based on composition.The YAM1 specimen, comprising 10% YAM by volume, underwent more profound etching over a small area, while YAM5, with the same proportion between two compositions, experienced deep etching across nearly half of the area.The lesser bonding energy of Al-O (512 kJ/mol) compared to that of Y-O (685 kJ/mol) underscores the significant role played by the reaction between Al-O bonding and fluorocarbon deposits [46][47][48] .This reaction results in the formation of AlF 3 layers, prone to removal via physical attack due to the vulnerability of fluorinated layers to ion sputtering.Consequently, an etching depth differential between the Y 2 O 3 and YAM compositions arose 15,25,32 .Craters within the Y 2 O 3 -YAM composite microstructure formed, but were very small in size, unlike the results of the Y 2 O 3 mono-composition.Sizes of craters formed during plasma www.nature.com/scientificreports/etching is intricately linked to the ceramic grain size.Hence, amalgamation of varying YAM and Y 2 O 3 compositions can reduce crater size and minimize changes in surface roughness if grain growth is effectually suppressed 49 .
For composition with little solidification at temperatures below the eutectic point, the pinning effect is much more effective at suppressing grain growth; it can dramatically reduce growth; resulting craters rarely form, and if formed, they are very fine 28 .In light of these considerations, a strategic combination of compositions recognized for their robust plasma resistance properties holds promise in effectively diminishing crater size, thereby attenuating changes of surface roughness and reducing contaminant particle generation.SEM images of the microstructure after plasma etching are shown in Fig. 7; the CF 4 :Ar gas ratio was varied for the YAM5 specimen with a 50:50 volume mixture of Y 2 O 3 and YAM ceramics.Overall, no significant differences in microstructure were seen with different plasma atmospheres, and there were no changes in composition of preferential etching.As the ratio of Ar gas increased, the number of craters formed on the surface with sizes of 1 µm or less increased, especially when the ratio of CF 4 :Ar:O 2 was 10:40:10, meaning that there was a very high amount of Ar, as shown in Fig. 7d; in this case, the effect of physical etching increased and more craters were formed.As shown in Fig. 7c, when the ratio of CF 4 :Ar:O 2 gas was 20:30:10, a large number of irregular nanopores formed on certain Y 2 O 3 grains 15 .
SEM-EDS analysis of the surface of the YAM5 specimen after etching in CF 4 :Ar:O 2 gas mixture with ratio of 40:10:10 is shown in Fig. 8. Different compositions exhibited different levels of etching resistance, as shown in Figs. 6 and 8a; the composition with darker colored grains and etched faster by the plasma is YAM composition, and the EDS mapping results confirms that the darker grains are YAM composition, as shown in Fig. 8b.Other compositions of Y 2 O 3 and F were detected throughout and no significant differences were found.In a recent study, bulk YAG ceramic was found to be etched slightly faster than bulk Y 2 O 3 ceramic under CF 4 :Ar:O 2 plasma gas ratios of 40:10:10 and 10:40:10 50 .This study shows that YAM ceramics also etch faster microscopically than Y 2 O 3 under the same conditions.
Next, the mechanical properties of the Y 2 O 3 and Y 2 O 3 -YAM composites were evaluated; results of Vickers hardness measurements are shown in Fig. 9.To serve as viable components in a practical semiconductor plasma etching apparatus, materials must have adequate mechanical properties alongside resistance to etching.The Y 2 O 3 ceramic is acknowledged for its inherently low hardness, typically within a range of 7-8 GPa; a low hardness value of 6.9 GPa was obtained for the specimen in this study 51 .The compromised mechanical properties of Y 2 O 3 pose impediments to its sustained use of ceramic components, prompting frequent replacement cycles.Therefore, enhancing the mechanical properties without losing plasma etching resistance is imperative.The pure YAM composition has excellent hardness of about 11 GPa 35 .By compounding YAM in Y 2 O 3 , the Vickers hardness improved and, as the proportion of YAM increased from 10 to 90%, the Vickers hardness improved to 9.2 GPa.Consequently, the composite of Y 2 O 3 and YAM did not deviate much from the Y 2 O 3 and Al 2 O 3 series of materials used as in-chamber materials for conventional semiconductor plasma etching processes, making it easy to apply in industry, while minimizing crater formation through grain growth inhibition.The excellent etching resistance, small surface roughness change, and enhanced mechanical properties are expected to enable the material to be used for a longer period and significantly improve the yield of semiconductor production, pushing it beyond the limitation of material components in plasma etching chambers that rely on conventional single composition and coating methods.

Conclusion
In conclusion, we studied the plasma etching characteristics of Y 2 O 3 -YAM composites and Y 2 O 3 under harsh environment with different CF 4 :Ar:O 2 mixed gases ratios.Y 2 O 3 polycrystalline ceramics showed fast etching rate, large change of surface roughness.In addition, there is formation of large craters at the surface after plasma etching.On the other hand, the Y 2 O 3 -YAM composite showed minimal surface roughness changes, while etching rate increased under the physical etching-dominant environment.Especially, under various gas conditions, composites with 50:50 volume fractions demonstrated superior physicochemical etching resistance compared to Y 2 O 3 ceramics.The composites also effectively reduced the size of the craters produced on the Y 2 O 3 surface after plasma attack.Based on these findings, Y 2 O 3 -YAM composites demonstrate remarkable inductively coupled plasma-reactive ion etching resistance in plasma etching conditions, accompanied by a notable decrease in the generation of contaminants.Furthermore, the low hardness, a critical drawback of conventional Y 2 O 3 ceramics,  www.nature.com/scientificreports/ is significantly enhanced in the composites.Characteristics have significantly improved without departing too far from the candidate materials previously used as components in semiconductor manufacturing process equipment.We contend that this study provides valuable perspectives for improving applications in the semiconductor manufacturing industry.

Following
the blending of Y 2 O 3 and YAM powders at varying volume ratios, the phases of the resultant composite ceramics, consolidated through hot-pressing at 1500 °C, were identified by XRD analysis, as shown in Fig. 1a.Notably, with as little as 10% YAM powder by volume, a predominantly cubic Y 2 O 3 (#86-1326) phase was identified, accompanied by a faint monoclinic YAM (#83-0935) phase.However, the intensity of the YAM phase peak relative to Y 2 O 3 increased as the proportion of the YAM phase increased.When Y 2 O 3 was more than twice as abundant as Al 2 O 3 , only YAM and Y 2 O 3 phases existed as phase diagrams in the pseudo-binary system during sintering at 1500 °C39 .Therefore, only YAM and Y 2 O 3 phases were detected in the composites sintered in this study, while phases such as Al 2 O 3 , Y 3 Al 5 O 12 and YAlO 3 were not identified.

Figure 1 .
Figure 1.(a) X-ray diffraction patterns of sintered Y 2 O 3 -YAM composite ceramics with different volume ratios.(b) Measured and relative density of sintered Y 2 O 3 -YAM nanocomposite ceramics with different mixed ratios.

Figure 3 .
Figure 3. Simple schematic illustration of plasma etching test chamber system.

Figure 4 .
Figure 4. Plasma etching rate of c-axis sapphire, Y 2 O 3 polycrystalline ceramics and Y 2 O 3 -YAM composites with different volume ratios under a variety of mixed gas ratios between CF 4 , Ar and O 2 conditions.

Figure 9 .
Figure 9. Vickers hardness of Y 2 O 3 polycrystalline ceramics and sintered Y 2 O 3 -YAM composites with different volume ratios.

Table 2 .
Details of plasma etching conditions.

Table 3 .
Mixed gas ratios for plasma etching test.range of 20° to 60°.The relative densities of all sintered samples were determined by the Archimedes method.The grain size was obtained by measuring the average line-intercept length of 150 grains.Average etching depths of sapphire, Y 2 O 3 , and Y 2 O 3 -YAM composites were assessed using a surface profiler (Tencor P-7 Stylus Profiler, KLA Co.) at three distinct positions for each specimen, employing scan length of 1 mm and scan rate of 200 Hz.